1. Field of the Invention
The invention relates generally to concrete finishing trowels and, more particularly, to finishing trowels that support an operator during use, i.e. riding trowels, with stabilizers for mitigating the effects of vibrations on trowel operation.
2. Description of the Related Art
A variety of machines are available for smoothing or otherwise finishing wet concrete. These machines range from simple hand trowels, to walk-behind trowels, to self-propelled riding trowels. Regardless of the mode of operation of such trowels, the powered trowels generally include one to three rotors that rotate relative to the concrete surface. Riding finishing trowels can finish large sections of concrete more rapidly and efficiently than manually pushed or guided hand-held or walk behind finishing trowels. The present invention is directed to riding finishing trowels.
More particularly, the invention relates to a concrete finishing trowel, such as a riding trowel, having rotor assemblies that can be tilted for a steering operation. Riding concrete finishing trowels of this type typically include a frame having a cage that generally encloses two, and sometimes three or more, rotor assemblies. Each rotor assembly includes a driven shaft and a plurality of trowel blades mounted on and extending radially outwardly from the bottom end of the driven shaft. The driven shafts of the rotor assemblies are driven by one or more engines mounted on the frame and typically linked to the driven shafts by gearboxes of the respective rotor assemblies.
The weight of the finishing trowel, including the operator, is transmitted frictionally to the concrete surface by the rotating blades, thereby smoothing the concrete surface. The pitch of individual blades can altered relative to the driven shafts via operation of a lever and/or linkage system during use of the machine. Such a construction allows the operator to adjust blade pitch during operation of the power trowel, typically by operating a crank mounted on a pitch control post and connected to the rotor assembly. As commonly understood, blade pitch adjustment alters the pressure applied to the surface being finished by the machine. This blade pitch adjustment permits the finishing characteristics of the machine to be adjusted. For instance, in an ideal finishing operation, the operator first performs an initial “floating” operation in which the blades are operated at low speeds (on the order of about 30 rpm) but at high torque. Then, the concrete is allowed to cure for another 15 minutes to one-half hour, and the machine is operated at progressively increasing speeds and progressively increasing blade pitches up to the performance of a finishing or “burning” operation at the highest possible speed—preferably above about 150 rpm and up to about 200 rpm.
The rotor assemblies of riding trowels also can be tilted relative to the vertical for steering purposes. By tilting the rotor assemblies, the operator can utilize the frictional forces imposed on the blades by the concrete surface to propel the vehicle. Generally, the vehicle will travel in a direction perpendicular to the direction of tilt of the driven shaft. Specifically, tilting the rotor assembly from side-to-side and fore-and-aft steers the vehicle in the forward/reverse and the left/right directions, respectively. It is also commonly understood that, in the case of a riding trowel having two rotor assemblies, the driven shafts of both rotor assemblies should be tiltable side-to-side for forward/reverse steering control, whereas only the driven shaft of one of the rotor assemblies needs to be tilted fore and aft for left/right steering control.
One problem experienced by all riding finishing trowels to one extent or another is undesired vibrations resulting from sliding contact between the rotating blades and the surface being finished. The causes of these vibrations are not completely understood. Nor is it fully understood why some sizes or brands of machines are more susceptible to these vibrations than others or why some abatement techniques are more effective than others. However, it is generally known that at least a major contributing factor to these vibrations is so-called “stick-slip vibration,” sometimes known as “chatter.” Stick-slip vibration is characterized by a saw-tooth wave of periodic cycles of motion and arrests and sometimes occurs between slowly moving bodies in dry or boundary lubricated sliding contact. When the moving body has a large contact surface, the stick-slip phenomenon is complex, especially when the body is rotating, due to the fact of the tangential velocity at a point in the surface varies with the radial distance from the axis of rotation. The distribution of the normal load over the surface also varies the multi-point loading pattern of the wake of the system over the rotating body. Chatter tends to increase with coefficients of friction and to decrease with contact pressure.
Generally speaking, midsize trowels such as 48″ trowels, i.e., those finishing a swath of the order of about 48″, are more susceptible to chatter than in 36″ trowels and 60″ trowels. Chatter tends to be the most pronounced when steel blades are employed rather than composite blades and blade pitch is set to be relatively flat—on the order of 0-5°. Chatter is also more pronounced when the coefficient of friction of the curing concrete is at a maximum, which occurs when the concrete is partially set but still has some viscosity. In addition, in any given trowel design, the vibrations tend to occur predictably at multiple, but repeatable on a cycle-by-cycle basis, rotor assembly RPMs. For instance, as a 48″ trowel accelerates from 0 to 150 rpms, it may experience chatter at 60, 100, and 125 rpm at a given blade pitch on a surface with a given coefficient of friction. These vibrations can become so severe in some machines that the entire machine “hops” up-and-down and side-to-side, resulting in considerable operator discomfort and, in some cases, marring of the concrete by the vibrating blades. Depending upon the make and size of the trowel, these vibrations can result in oscillation of the top of the pitch control post of 2″ or more. These effects could be reduced by increasing blade pitch to increase pressure, but that is not an option on relatively soft concrete or concrete having imbedded fibers that might be cut by or snagged on a highly-pitched blade
In any mechanical system, vibrations can be reduced by increasing the system's stiffness (hence increasing its spring constant), or damping the system. Prior attempts to reduce chatter focused primarily on increasing the system's stiffness. For instance, Whitemen reduced chatter in its finishing machine, as measured by oscillation of its pitch control posts, to about 1.5″, presumably by maximizing the stiffness of its frame and other trowel components. However, these measures came at the costs of increased weight and expense and would require a substantial redesign of other trowels. Blades made of composite plastics have also been introduced and have been quite effective at reducing chatter because they have a much lower spring constant than traditional steel blades as well as a lower coefficient of friction. However, these blades are substantially more expensive than steel blades and have met with limited industry acceptance.
Accordingly, there is a need for a ride-on concrete finishing trowel that experiences less vibrations during operation than traditional ride-on concrete finishing trowels.
The need also exists to provide a stabilizing system for a ride-on concrete finishing trowel that is non-intrusive and simple and inexpensive to construct and install.